1. Field of the Invention
The present invention relates to techniques for communication using time-varying electric fields.
2. Description of the Related Arts
In recent years, methods have been proposed for performing communications using electrostatic fields induced in a dielectric such as the human body. This method was first disclosed in T. G. Zimmerman's paper “Personal Area Networks: Near-Field Intra-Body Communication” (IBM System Journal Vol. 35, No. 3&4, 1996-MIT Media Laboratory). According to this method, it is possible to implement a miniaturized device permitting a reduction in the electric power required of the device.
However, in communications based on the method, so-called PANs (Personal Area Networks), there has been the problem of establishing return paths. As shown in FIG. 22, a PAN can use earth ground as a return path. For this reason, there is a need to establish an electrostatic coupling through the earth ground between the transmitting apparatus and the receiving apparatus. Accordingly, when establishing the transmitting and receiving apparatus in a location that separates the transmitting and receiving apparatus from the ground, the electrostatic coupling weakens, and stable communications becomes impossible. As a result, electric field communications apparatus based on PANs has been considered capable only of communicating across limited distances.
To solve the earth-ground difficulty, Japanese Patent Laid-Open Publication No. 10-229357 and Japanese Patent Laid-Open Publication No. 2001-298435 disclose technology that has the aim of extending the communication distance. These disclosed technologies have in common the point of attempting increases in the communication distance by using electrostatic coupling through the air other than earth ground as the return paths.
FIG. 23 through FIG. 26 shows in general the communication theory of electric field communications apparatus that communicate using electrostatic coupling through the air as a return path.
The transmitter apparatus in FIG. 23 outputs as a time-varying voltage between electrode ERBT and electrode ERGT, a signal modulated based on data to be transmitted. Then, it generates a voltage difference with electrode ERBT and electrode ERGT, thereby creating an electric field. In general, it is relatively easy to transmit an electric field in a dielectric such as the human body compared with the air. Accordingly, when electrode ERBT is directly contacted to a dielectric such as the human body as shown in FIG. 24, it is possible to make electric fields reach a greater distance. As well, as shown in FIG. 25, when the receiver apparatus is positioned inside the electric field generated by the transmitter apparatus, it gives rise to a voltage difference between the electrodes ERBR and ERGR of the receiver apparatus. The receiver apparatus detects the voltage difference, and can acquire the transmitted data by demodulating the voltage difference. In this case, the return path is the electrostatic coupling established through the air between the electrodes ERGT and ERGR of the transmitter apparatus. As shown in FIG. 26, a dielectric may be used as a return path. In this case, the dielectric return path extends the communications distance for the electric field communications apparatus.
Each of the technologies in the two Patent documents above can solve the earth-ground problem.
However, even technologies disclosed in the Patent documents cannot assure sufficiently long communication distances. This reason will be described below. In Japanese Patent Application Laid-Open Publication No. 10-229357, in order to solve the earth-ground problem, the return path by electrostatic coupling through the air is assured by pointing the transmitter return electrode and the receiver return electrode toward the air. However, since this is for electrostatic coupling through the air, the distance between the return path of the transmitter apparatus and the receiver apparatus must not be overly increased. In the case of performing electric field communications using the configurations disclosed in the Patent documents, it becomes impossible to communicate across a space as wide as the space between a person's head and pelvic region.
With the technology disclosed in Japanese Patent Application Laid-Open Publication No. 2001-298425, the return electrode is removed, and a cabinet made of conductive members substitutes for the return electrode. In this technology, a high-sensitivity electric field sensor is used for measuring the electric field. An electro-optical device that exhibits the so-called Pockels Effect is used as an electric field sensor. The electric field sensor is also capable of measuring smaller changes in an electric field, compared to one using a transistor or an FET (Field-Effect Transistor) or the like. However, by using a configuration that combines the functions of cabinet and return path, it is not clear how an electric field reaching the receiver apparatus is specifically distributed in the interior of the apparatus. For example, in a case that only a very small portion of the electric field reaches the part in which the electric field sensor is positioned, sensitivity to changes in the electric field cannot be improved. In other words, using the technology disclosed in the patent documents, it is difficult to sufficiently raise the reception sensitivity because the electric field density at the location of the electric field sensor cannot be strictly estimated.